docs: def key2str(target):
      """
      In ``symfit`` there are many dicts with symbol: value pairs.
      These can not be used immediately as \*\*kwargs, even though this would make
      a lot of sense from the context.
      This function wraps such dict to make them usable as \*\*kwargs immediately.
  
      :param target: `Mapping` to be made save
      :return: `Mapping` of str(symbol): value pairs.
      """
      return target.__class__((str(symbol), value) for symbol, value in target.items())